The present invention relates to the field of electronic tuning systems.
A variety of electronic tuning systems are known for tuning radio and television receivers in which the magnitude of a tuning voltage determines the frequency to which a voltage controlled tuner is tuned. Electronic tuning systems may be generally categorized as belonging to one of three groups: (1) frequency synthesis type systems including a closed loop for generating the tuning voltage in which the frequency of the local oscillator signal generated by the tuner is compared to a reference signal; (2) memory type systems including a memory for storing binary signals representing the magnitude of the tuning voltage for each channel which may be selected and digital to analog converter for converting the binary signals to the tuning voltage of appropriate magnitude when a respective channel is selected; and (3) search type systems for generating a tuning voltage having a ramp-like magnitude characteristic to sweep the frequency to which the tuner is tuned until an RF carrier with acceptable reception characteristics is located.
Frequency synthesis type tuning systems are relatively accurate, but tend to be relatively expensive because they generally include relatively high speed frequency dividers. Memory type tuning systems are less expensive than the frequency synthesis type tuning systems but tend to be less accurate since memory type tuning systems are essentially open loop systems. Search type tuning systems are generally less expensive than frequency synthesis type tuning systems since they do not require high speed dividers and are generally more accurate than memory type tuning systems because of their closed loop nature.
However, search type tuning systems compare less favorably to frequency synthesis and memory type tuning systems with respect to ease of channel selection. Specifically, in frequency synthesis and memory type tuning systems, the tuning process generally starts with the generation of binary signals representing the selected channel. Therefore, channel selection apparatus for directly selecting a channel, e.g., by depressing digit keys of a keyboard corresponding to the tens and units digits of the associated channel number, is readily incorporated in frequency synthesis and memory type tuning systems. In conventional search type tuning systems, the tuning process generally starts with the location of an RF carrier with acceptable reception characteristics. Therefore, search type tuning systems are not particularly well suited for direct channel selection.